1. Field of the Invention
This invention relates to a spark plug having an electrode whose front end has a spark-erosion resistant tip made from a ruthenium- or iridium-based metal in which an oxide of a rare earth metal group is dispersed.
2. Description of Prior Art
In a spark plug electrode, a firing tip is introduced which is made from a high melting point metal such as ruthenium or iridium or the like. In the metal, an oxide (yttria) of a rare earth metal group is dispersed in order to improve a spark-erosion resistant property as shown by Japanese Patent Publication No. 52-118137.
In Japanese Patent Publication No. 2-49388, a firing tip is secured to a front end of a nickel-based electrode by means of laser or electron beam welding. The firing tip is made of an iridium-based metal containing platinum in less than 50% by weight.
The laser or electron beam welding causes to locally apply thermal energy to the firing tip and the front end of the electrode so as to form a solidified alloy layer therebetween. In this instance, the oxide of the rare earth metal group tends to coagulate or segregate in the solidified alloy layer so as to appear blow holes. This tendency becomes more remarkable as the oxide in the firing tip increases.
These blow holes cause thermal stress to develop cracks in the solidified alloy layer due to heat and cool cycles when the spark plug electrode is applied to an internal combustion engine. At the worst case, the cracks eventually leads to exfoliate or fall the firing tip from the front end of the electrode to significantly shorten a service life of the spark plug.
In order to avoid the exfoliation of the firing tip, it is considered to decrease an amount of the oxide of the rare earth metal group. However, the decrease the amount of the oxide results in declining the firing tip of the spark-erosion resistant property.
Therefore, it is an object of the invention to provide a spark plug which is capable of reducing a spark discharge voltage, and preventing blow holes and cracks from occurring in a solidified alloy layer between a firing tip and a front end of an electrode without inviting a loss of the spark-erosion resistant property.